The present invention relates generally to global positioning system (GPS) receivers and, more particularly, to the control of configuration or operating parameters for such receivers.
GPS receivers are designed to provide estimates of a user""s position on or near the surface of the earth based on ranging measurements to orbiting satellites. Performance capabilities of such receivers are primarily affected by two factors. First, satellite geometry, which causes geometric dilution of precision, and second, ranging errors.
Ranging errors are generally organized within six major areas including errors due to satellite ephemeris information, satellite clock errors, ionospheric group delay, tropospheric group delay, multipath errors, and receiver measurement errors. Modern GPS receivers typically employ processing techniques to reduce or eliminate the effects of these errors.
Geometric dilution errors can be calculated for any instantaneous satellite configuration as seen from a particular user""s location. It has been determined that for a 21-satellite constellation and a three-dimensional position fix, the world-wide median value of the geometric dilution factor is approximately 2.7. This quantity is usually called the position dilution of precision (PDOP). Typical usable PDOP factors range from approximately 1.5 to approximately 8. In general, PDOP factors may vary between 1.0 and ∞, however, most users will not accept positions computed with a PDOP of greater than approximately 15. Variations in this PDOP factor are typically much greater than the variations in ranging errors.
Using these error factors, the root mean square (RMS) position error provided by a conventional GPS receiver may be expressed as follows:
RMS position error=geometric dilutionxc3x97RMS ranging error. See, e.g., Bradford W. Parkinson and James J. Spilker Jr., Global Positioning System: Theory and Applications, Vol. 1, p. 16 (1996). Thus, the lower the geometric dilution factor (usually PDOP for terrestrial applications), the better the position estimate that a GPS receiver is able to provide.
Understanding the effect PDOP (or any one of a variety of other parameters for that matter) has on the accuracy of position estimates provided by a GPS receiver is important; especially considering that conventional GPS receivers typically do not utilize all of the satellite data which they receive to derive a position estimate. Instead, most GPS receivers employ masks or filters to select only data from those satellites which satisfy certain selection criteria (e.g., minimum elevation above the horizon) to derive a position solution. For example, filters or masks may be used to ensure that a desired maximum PDOP is permitted.
The use of such filters is important because, to achieve a positioning accuracy to a given requirement (sayxc2x11.0 meter), the ranging accuracy and satellite geometry must both be within acceptable tolerances. For example, if individual ranging measurements to satellites have statistically independent error of zero mean, then PDOP becomes a direct multiplier in determining position error. Generally, if PDOP rises above six, it is an indication that satellite geometry is not very good from the user""s stand point, and significant position errors can be expected. By controlling the limit on the maximum acceptable PDOP then, a GPS receiver can be configured to provide position fixes to a desired degree of accuracy.
In addition to PDOP, many other factors influence the relative accuracy of a position determination made by a GPS receiver. Among these factors are the number of satellites used to compute the position fix, the relative signal to noise (SNR) strength of the data signals received from those satellites, satellite geometry and cutoff elevation. In addition, the optimal configuration of operating parameters is also a function of both the work environment and the user""s application. For example, the optimal configuration for a city environment is different from a rural environment. Furthermore, one user""s application may require a fast acquisition and determination of location while another may require a highly precise determination of position, regardless of the amount of time required to determine the position.
In the past, some conventional GPS receivers have allowed users to modify some of these configuration parameters individually, in order to allow the user to customize the receiver for a given application or environment. However, knowing which receiver parameters to adjust, and in what fashion, typically requires knowledge about the satellite data signals which are currently being received. In most cases, users either do not have such information or are not familiar enough with the operation of the receiver to make an intelligent decision about which configuration parameter(s) to adjust and how. Accordingly, what is needed is a means of easily adjusting the configuration parameters of a GPS receiver to achieve a desired degree of accuracy in the position estimates provided thereby.
The present invention provides, in one embodiment, a control system for a GPS receiver that allows users to easily and simultaneously adjust multiple data collection (i.e., configuration) or operating parameters for the receiver. Thus, if a user is unsuccessful in obtaining a position fix with a given set of receiver configuration parameters, those parameters may be quickly modified to allow the user to obtain a position fix.
In another embodiment, the present invention provides a method of setting configuration parameters for a GPS receiver in response to user input specifying a desired quality of position estimate to be provided by the receiver. The user input may be received using a slide bar control, which may be a graphical representation displayed to the user. The slide bar control may allow the user to choose from a number of predetermined settings, each corresponding to a set of GPS receivers configuration parameters. Distribution of the settings over the range of the slide bar control need not be linear. Preferably, each set of the configuration parameters includes a setting for a PDOP mask, an SNR mask, an elevation mask and a minimum number of satellites to be used by the receiver in making a position compensation. In other embodiments, other combinations of configuration parameters may comprise a set.
In an alternate embodiment, the present invention provides an apparatus and method for setting parameters to use for position estimates in a combination GPS and Pseudolite receiver.
These and other features and advantages of the present invention will be apparent from the detailed description and its accompanying drawings which follow.